Gas-Phase Retro-Diels-Alder Reactions of Cyclohexene, 1-Methylcyclohexene, and 4-Methylcyclohexene following Photoexcitation at 193 nm: A Velocity-Map Imaging Study

We present the results of a velocity-map imaging study into the retro-Diels-Alder (RDA) reactions of cyclohexene, 1-methylcyclohexene, and 4-methylcyclohexene following photoexcitation at 193 nm. Universal detection of all neutral fragments via vacuum-ultraviolet photoionization at 118 nm allows imaging of both RDA fragments in all cases. Fragment kinetic energy distributions reveal contributions from both dissociative ionization and retro-Diels-Alder reaction of the respective parent molecules, yielding reaction products with a high degree of internal excitation. Together with the observed isotropic product angular distributions, this is consistent with a mechanism in which the RDA reaction occurs from high vibrational levels of the electronic ground state following internal conversion from a higher-lying state initially populated in the photoexcitation process, as predicted by frontier molecular orbital theory. Velocity-map images and total translational energy distributions for the RDA products of 1-methylcyclohexene and 4-methylcyclohexene are very similar to those for unsubstituted cyclohexene, indicating that methyl substitution either adjacent to or far from the double bond has little effect on the dynamics of the RDA process.